Wednesday, 20 April 2016

Concrete is the most widely used construction material in the world as its ingredients, e.g. aggregate, water and cement, are readily available everywhere. The properties of hardened concrete are governed by various factors such as the mixture design and curing conditions. Concrete is often ordered for a project based on the required strength. This is a critical parameter as various operations such as formwork removal and application of load depend on when the concrete reaches a specific strength.

The most commonly used method to monitor the strength of concrete is through breaking specimens that are poured and cured in the field subjected to the same curing conditions as for the concrete structure. Sometimes, cast-in-place concrete specimens are prepared by embedding plastic molds in the slab to achieve a better representation of real concrete strength.

This method is not efficient as there is a delay in obtaining results (through the time spent of sending samples to laboratory, breaking, and reporting), and is not accurate as usually one specimen is tested. Even a couple of hours of delay and uncertainty in results has significant financial implications as the construction crew is waiting on the job site.

Among other non-destructive methods of strength measurement, the pull-out and Schmidt hammer tests are not reliable as they greatly influenced by local material conditions. The only non-destructive method that is widely accepted by the standards and building codes is the concrete maturity test. In this method, the strength is correlated with the concrete maturity which is a function of temperature history in field-cured concrete. ASTM C1074 and NEN 5970 describe three different standard specification for this method.

The in situ temperature monitoring which is the key part of maturity method, is often performed using wired loggers or thermocouples connected to a data logger. These wired systems are installed prior to pouring concrete. After hardening, the temperature data is collected either using a hand-held data logger or connecting the logger-box to computer. This data is then analyzed based on the concrete maturity concept to calculate the concrete strength. One of the challenges of using these systems is to take care of the wires as they often get cut in the construction job site. Moreover, the data collection and analysis is time-consuming.

Recently, wireless mobile-based sensors (such as Giatec SmartRock) have been developed for more efficient approach to concrete temperature/maturity monitoring. Utilizing these wireless mobile-based smart sensors can significantly reduce the time and accordingly the cost of concrete quality control. Here is an example calculation of the cost saving based on a real project experience.

In a typical 40-story high-rise concrete structure, 200 maturity meters are needed for real-time strength monitoring. Using each wireless temperature sensor helps saving 20 minutes of installation time, and 40 minutes of reading/analyzing time. Assuming that the data from each sensor is collected at 3 different times by a technician with hourly cost of $50, the total labor cost saving in the quality control of this project will be about $63,500. This saving easily justifies the investment of $17,000 for wireless SmartRocks ($85 each) instead of using wired temperature sensors.

In addition, there is a significant cost saving as a result of using maturity method by eliminating the need to break samples (about $30K to $50K in the above example project) and more importantly by optimizing the construction schedule as the real-time strength monitoring enables project managers to save up to a day on early formwork stripping for each floor. Even in projects where the break test is mandatory, the maturity concept can be used to determine the optimum time at which the field specimens should be sent to lab for the break test. Considering these advantages, it is no surprise that more and more contractors are implementing maturity method in their engineering practice. See some of the examples here where SmartRock sensor has helped improving the project flow of various companies worldwide.